Gas turbines may have an expansion joint positioned between a turbine duct flange and a diffuser duct. The diffuser duct provides performance benefits to the turbine as a whole by expanding the exhaust gases to achieve optimum aerodynamic pressure recovery. Most turbine ducts run hot and are machined structures while most diffuser ducts are lower cost fabricated casings that are internally insulated and relatively cold. Because of the thermal mismatch at this connection, an expansion joint is generally used to accommodate the large relative displacements between these components.
Known expansion joints can take many forms. When relatively large axial, vertical, and lateral movements are expected, the expansion joint may be designed with a vertically mounted flexible element. This type of design requires a vertical offset between the turbine duct aft flange and the diffuser duct forward flange. The offset provides a location to attach each end of the flexible vertical element and also provides a collection trough for water wash fluid and/or liquid fuel that may enter the diffuser after a false start.
The open trough, however, presents a discontinuity in the flow path and may result in a pressure loss that negatively impacts the overall turbine performance and heat rate. Further, liquid fuel that does not fully drain from the combustion system will flow into the exhaust diffuser duct. If not properly drained, the liquid fuel may soak the insulation and become a fire hazard when the gas turbine does start. Likewise, water from turbine water wash may enter the expansion joint insulation and flow out onto the ground. To address the drainage issue, a flow shield has been bolted to the turbine flange so as to protect the flexible element. The flow shield, however, must face high transient thermal stresses and does not provide total protection from pressure pulsations. As a result, the flow shield generally must be fabricated with high cost materials.
There is a desire, therefore, for an improved turbine expansion joint that will allow drainage of liquids entering the exhaust diffuser duct while limiting damage to the internal insulation. The expansion joint preferably provides a smooth aerodynamic transition between the ducts. Moreover, the expansion joint preferably may be made with low cost manufacturing methods and materials that are easy to install and maintain.